This invention relates to communications, and more particularly relates to methods for maintaining the voice path connection during a caller disconnect of an emergency 911 call.
Wireless local loop systems represent a significant improvement in telephone service by providing customers alternative access routes to public telephone networks. Users can continue to use familiar and convenient conventional telephone equipment yet communicate over a wireless communication link. A customer premises radio unit (CPRU) provides the interface between the conventional telephone equipment and the wireless communication link.
The use of conventional phones coupled to a wireless communication link presents a problem, however, because conventional phones receive a dial tone from the Central Office (CO) when the user takes the telephone handset off-hook. In wireless phones the user typically has no dial tone. Rather than hearing a dial tone generated by the CO before dialing, wireless dialing convention dictates that the user enter the desired telephone number and press a SEND button or its equivalent. Only after the SEND button is pressed does the wireless phone begin acquiring a communication channel.
West et al., U.S. Pat. No. 4,658,096, disclose a system for interfacing a conventional phone to a wireless communication link. This system simulates a dial tone when the user goes off-hook. It also performs digit analysis on the dialed number to determine when the last digit has been dialed, storing the dialed digits during this process. After digit analysis indicates when the last digit has been dialed, the system generates a SEND signal transmitting the stored digits to the telephone network.
The above-described technique has a particular disadvantage in that it does not provide a digit analysis that can accommodate changes in numbering schemes. Further, this interface unit requires that the telephone numbering algorithms of the ultimate locale where the interface unit will be used be programmed into the interface unit. As an example, different countries have different numbering formats for the initiation of phone calls. Thus, an interface unit that operates in one country according to the numbering scheme of that country will not necessarily operate properly in another country with a different numbering scheme. Moreover, as the number of telephone users increases, new numbering schemes will be devised to accommodate them. It will be inconvenient and costly to reprogram the digit analysis in interface units already deployed.
Bilgic, et al., U.S. Ser. No. 08/676,975, filed Jul. 8, 1996, the contents of which are hereby incorporated by reference, disclose systems and methods for interfacing conventional phone equipment to a wireless communication link wherein the digit analysis is performed at the wireless base station controller rather than at the CPRU. This allows relatively easy changes to the digit analysis algorithm to accommodate changes in telephone numbering schemes. Because the CPRUs disclosed by Bilgic et al. are robust to numbering scheme changes, these CPRUs are denoted as universal radio controllers (URCs). Not performing digit analysis, the URC transmits the dialed digits to the base station substantially at the same time as they are dialed (denoted Overlap Sending if the digits are sent out-of-band).
Despite the advances demonstrated by Bilgic et al., there remain proposed wireless local loop systems in which digit analysis is performed neither at the base station nor at the CPRU. Instead, these systems perform the digit analysis at the telephone network central office. In such systems, because the CPRU and base station perform no digit analysis, an emergency call is currently treated as an ordinary outgoing call by the CPRU. If the CPRU adheres to DECT (Digital Enhanced Cordless Telecommunications) protocol, all calls, including emergency 911 calls, will be torn down within the wireless network after the user places the handset on-hook or otherwise disconnects. Thus, when a user goes on-hook, the CPRU will begin normal call teardown procedures (by, e.g., sending a CC_RELEASE message with a reason of normal). This presents a problem, however, because the essential element of emergency 911 calls is that the call control (i.e., the ability to disconnect the callxe2x80x94termed Disconnect Supervision) is taken away from the calling CPE party.
For normal calls, the caller and the called party share call control (Disconnect Supervision) in that either party has the power to go on-hook and have the call torn down. In an emergency call, should the caller go on-hook, the public safety answering point (PSAP) then re-rings the on-hook user to reconnect the user. In the wireless local loop environment, such reconnection may not be possible because of the scarcity of available bandwidth and other factors. In addition, the reconnect notification originating at the CO, using a GR-303 interface (Bellcore""s GR-303 defines a set of requirements for Integrated Digital Loop Carrier (IDLC) systems) does not contain the telephone number of the emergency caller. Instead, the reconnect notification message contains only alerting information. Under GR-303, the CO is not aware of anything other than a wired line connection and therefore will not have any knowledge about an air link interface and any extra information that would be needed to convey information to the CPRU to reconnect the call.
Thus, there is a need for improved methods for maintaining a voice path connection over a wireless local loop to an emergency caller who has disconnected because the CPRU did not have knowledge of the type of call being placed.
In one embodiment of the invention, the CPRU is informed during call origination that it will not have call control because the caller is placing an emergency call. This may be accomplished by performing a digit analysis in the CPRU or at the wireless switch (i.e., radio node controller (RNC)). The digit analysis need only identify the digit string corresponding to the emergency call number and may ignore additional digits dialed thereafter. Should the CPE go on-hook during an emergency 911 call, the CPRU may immediately alert the CPE with a predetermined alerting tone, and apply ringback to the far end (of the PSAP).
In an alternate embodiment, neither the CPRU nor the radio node controller (RNC) need identify whether the call being placed is an emergency call. Instead, when the CPE goes on-hook during a call, the CPRU places itself into a Pending Disconnect state but does not release the communication link. Thus, the Pending Disconnect state allows for a possible reconnection of the existing call. Because the CO has performed digit analysis, the CO has identified whether the call is an emergency call. In the event of an attempted emergency call disconnect by the CPE, the central office (CO) sends alerting information to the RNC that is then relayed to the CPRU indicating the ringing tone to be applied to the customer premises equipment (CPE). Two-way voice traffic may resume when the CPE goes back off-hook.